This application claims the benefit of Korean Patent Application No. 2002-20269 filed on Apr. 13, 2002, in the Korean Industrial Property Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to microwave ovens, and more particularly, to a microwave oven which controls a variation in a cooking time period in accordance and proportionally with the quantity of food being cooked.
2. Description of the Related Art
Generally, microwave ovens are machines which cook food with the assistance of a variety of atmospheric sensors, such as a humidity sensor, a temperature sensor and a gas sensor, in addition to a weight sensor which measures the weight of the food to be cooked. A conventional cooking operation of such a microwave oven is described below.
To initiate a cooking operation, a user operates a start button of the microwave oven after laying the food on a turntable-type cooking tray installed in a cooking cavity of the microwave oven, and selecting a desired cooking mode in an automatic cooking menu provided on a control panel of the microwave oven. Thereafter, a microprocessor of the microwave oven receives a signal output from a humidity sensor of the microwave oven, and compares the signal output from the humidity sensor with preset reference data stored in a data storage unit of the microwave oven. Then, the microwave oven calculates a target cooking time period so as to control a magnetron of the microwave oven in accordance with the calculated target cooking time period.
In a conventional method of controlling the microwave oven, a first cooking time period is determined such that it is terminated at a time when a calculated slope of a sensor output value becomes equal to a preset reference slope. A second cooking time period is determined in accordance with the first cooking time period and factors preset in accordance with the kind of food to be cooked. When the second cooking time period expires, the total time period of the cooking operation ends.
In the conventional method, current atmospheric conditions obtained from signals output from the humidity sensor, temperature sensor and gas sensor during the cooking operation are different from previous atmospheric conditions. Therefore, the slope of a sensor output curve varies, and it is difficult for the microprocessor of the microwave oven to determine a precise first cooking time period. As such, it is necessary in the conventional method of controlling the microwave oven to provide an initial standby time period of, for example, about 20 minutes, before starting the magnetron of the microwave oven in a new cooking operation. During such an initial standby time period, the magnetron is stopped, and a fan installed in a machine room of the microwave oven is operated to reduce the temperature in the cooking cavity to near a predetermined point.
FIG. 1 shows a graph expressing a conventional method of controlling a cooking operation of the microwave oven described above. As shown in the graph, the method includes sectioning a total cooking time period into an initial standby time period TC, a first cooking time period T1, and a second cooking time period T2. That is, at an initial stage of the cooking operation, in a selected cooking mode, the temperature in a cooking cavity is reduced to near a predetermined point during the initial standby time period TC of, for example, about 20 minutes. The first cooking time period T1 starts at a time when the initial standby time period TC ends, and is terminated when a calculated slope of a sensor output value becomes equal to a preset reference slope xe2x80x9cA.xe2x80x9d
The first cooking time period T1 is determined in accordance with the quantity of food to be cooked. In such a case, measuring of the quantity of the food may be directly performed using a weight sensor. However, to avoid the use of expensive weight sensors in the microwave ovens, the measuring of the quantity of the food may be performed through an indirect method using an inexpensive humidity sensor. That is, the microprocessor of the microwave oven may measure humidity or the amount of moisture, i.e., in a form of steam, laden in air generated and discharged from the cooking cavity, and determine the quantity of food from the measured humidity.
To allow the microprocessor to determine the first cooking time period during the cooking operation, reference data of relationships between the amounts of food and humidity of discharged air is experimentally obtained from several cooking operations of specified foods, and stored in a data storage unit. Accordingly, the microprocessor controls the cooking operation of the food using the reference data stored in the data storage unit. Additionally, the first cooking time period T1 is used as a variable when determining the second cooking time period T2. That is, the second cooking time period T2 is determined in accordance with both the first cooking time period T1 and factors preset in accordance with the kind of food to be cooked.
Therefore, when a cooking operation is started, the microprocessor primarily determines a first cooking time period T1 in accordance with the quantity of food to be cooked. After the determination of the first cooking time period T1, the microprocessor determines another time period needed to complete the cooking after a termination of the first cooking time period T1, and sets the determined time period as a second cooking time period T2. In such a case, the determination of the second cooking time period is accomplished by searching the reference data, which is stored in the data storage unit and indicates the relationship between the first and second cooking time periods. When the second cooking time period T2 is completed, the cooking operation is terminated.
In some of the models of the conventional microwave ovens, the transition from the first cooking time period T1 to the second cooking time period T2 is determined in accordance with output values of humidity sensors. Particularly, the above transition is determined by a characteristic curve of a sensor output value indicating sensed humidity (%) of discharged air, as a function of time. In the conventional control method of the microwave ovens, such a reference slope xe2x80x9cAxe2x80x9d is set by a slope of the characteristic curve at a point where the sensor output value, indicating sensed humidity of discharged air, initially exceeds a preset reference value. The above-mentioned preset reference value is experimentally obtained. That is, the preset reference value is set at a point of the characteristic curve of the sensor output value where the slope of the curve rapidly changes, ideally at a point with a slope of xe2x80x9cA.xe2x80x9d
However, even though the total cooking time period (T1+T2) can be automatically determined using the stored reference data in accordance with the quantity of food as described above, users may want to lengthen or shorten the cooking time periods to cook foods for periods of time which are longer or shorter than the automatically determined cooking time periods. In other words, some users may prefer lesser cooked food rather than medium- or well-done food, and may desire to shorten the cooking time period. On the other hand, others may prefer well-done food rather than the medium- or lesser cooked food, and may want to lengthen the cooking time period.
To allow the users to adjust the cooking time period of a cooking operation, which is different from the automatically determined cooking time period, the conventional microwave oven is provided with a mode-selecting unit through which the users adjust the cooking time period. Accordingly, when a user inputs a desired cooking mode by manipulating the mode-selecting unit, the cooking time period automatically determined in accordance with the quantity of food is lengthened or shortened, so as to cook the food to a user""s taste.
FIG. 2 shows a graph illustrating characteristic curves of three types of cooking modes, that is, a high mode 204 with a longer cooking time period, a standard mode 202 with a standard cooking time period, and a low mode 206 with a shorter cooking time period, which are selected by a user through the mode-selecting unit. To cook 100 g of food using a conventional microwave oven, a first cooking time period T1a is primarily determined in accordance with the quantity of the food. Thereafter, as shown in the drawing, a second cooking time period T2a is determined in accordance with the first cooking time period T1a. That is, the standard cooking time period is determined as (T1a+T2a) by summing the first and second cooking time periods. When a user manipulates the mode-selecting unit to adjust the cooking time period in accordance with his/her taste, the standard cooking time period (T1a+T2a) for 100 g of food may be lengthened or shortened by xcex94ta.
To cook 300 g of food using the conventional microwave oven, a first cooking time period T1b is primarily determined in accordance with the quantity of the food. Thereafter, a second cooking time period T2b is determined using the first cooking time period T1b. Therefore, the standard cooking time period is determined as (T1b+T2b) by summing the first and second cooking time periods. When a user manipulates the mode-selecting unit to adjust the cooking time period in accordance with his/her taste, the standard cooking time period (T1b+T2b) for 300 g of food may be lengthened or shortened by xcex94tb.
As shown in FIG. 2, the characteristic curve of the high mode 204 is positioned above the characteristic curve of the standard mode 202, and the characteristic curve of the low mode 206 is positioned below the characteristic curve of the standard mode 202. Therefore, it is noted that the total cooking time period in the high mode 204 is longer than that of the standard mode 202, and the total cooking time period in the low mode 206 is shorter than that of the standard mode 202, even though the three modes 202, 204 and 206 have the same first cooking time period T1.
As further shown in FIG. 2, the slopes of the characteristic curves of the three modes 202, 204 and 206, which are determined on the basis of the quantity of food, are equal to each other. Such an equal slope of the characteristic curves of the three modes 202, 204 and 206 is caused by the fact that the second cooking time period T2 of the high mode 204 or the low mode 206 is lengthened or shortened by the same period of time regardless of a variation in the first cooking time period T1 determined on the basis of the quantity of the food. This means that the variations xcex94ta and xcex94tb in the second cooking time periods T2a and T2b of the two modes 202 and 204, which are performed in response to a user""s input signals transmitted from the mode-selecting unit, are always constant notwithstanding a difference in the quantity of foods to be cooked in cooking operations.
Since the total cooking time periods of the corresponding cooking operations of a conventional microwave oven are lengthened or shortened by the same period of time regardless of a difference in the quantity of foods, for example, 100 g and 300 g, as described above, it is very difficult for a user to prepare the foods of different quantity to his/her desired taste. That is, when a user wants the 100 g food to be well-done, a lengthened cooking time period by 30 seconds may be sufficient for the 100 g of food. But that same additional 30 seconds may not be adequate to prepare well done food for the 300 g of food.
However, since the cooking time periods in the conventional microwave oven are lengthened or shortened by the same time period, regardless of a difference in the quantity of foods, the conventional microwave oven fails to provide a cooking operation suited for an individual""s taste.
Accordingly, it is an object of the present invention to provide a microwave oven which is designed to allow a user to set a cooking time period such that the cooking time period is controllably lengthened or shortened in proportion to the quantity of food to be cooked.
Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
To achieve the above and other objects of the present invention, there is provided a microwave oven which performs a cooking operation in one of cooking modes having a first cooking time period and a second cooking time period, comprising a cooking cavity and a sensor which senses a state of air in the cooking cavity, wherein the first cooking time period is determined in accordance with an output value of the sensor, the second cooking time period is determined in accordance with the first cooking time period such that the first and second cooking time periods are expressed by a functional relation, and the cooking modes include a standard mode in which the second cooking time period is a standard second cooking time period, a high mode in which the second cooking time period is lengthened from the standard second cooking time period, and a low mode in which the second cooking time period is shortened from the standard second cooking time period. The second cooking time period of the high mode is varied to increase in proportion to the first cooking time period, and the second cooking time period of the low mode is varied to increase in inverse proportion to the first cooking time period.